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Ran fine last fall, now overheating. Need ideas

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fastformula

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The engines are 500HP's which are the blue Mercury racing 502's. Ran the boat last fall, everything was fine. First time out this year the high temp alarm went off. I changed stats and still having temp issues. There are a couple other strange things going on. The water pressure on both engines always ran about 10 to15lbs at speed. Now both engines bury the needle at 30lbs at speed. Also, the temp gages will hang at 100 degrees, then drift up to 180, then drift back down to 100. The gauge does not read hot, but the alarm gets triggered. I replaced the impellers last fall, and there is plenty of water exiting the exhaust tips. The engines have Hardin exhaust manifolds with jacketed thru hull risers. I have changed thermostats, but both engines still have high water pressure and gauge fluctuation. One engine still sets the high temp alarm. I am a pretty proficient mechanic, but this has got me stumped. Any suggestions ?
 
fastformula said:
The engines are 500HP's which are the blue Mercury racing 502's. Ran the boat last fall, everything was fine. First time out this year the high temp alarm went off. I changed stats and still having temp issues. There are a couple other strange things going on. The water pressure on both engines always ran about 10 to15lbs at speed. Now both engines bury the needle at 30lbs at speed. Also, the temp gages will hang at 100 degrees, then drift up to 180, then drift back down to 100. The gauge does not read hot, but the alarm gets triggered. I replaced the impellers last fall, and there is plenty of water exiting the exhaust tips. The engines have Hardin exhaust manifolds with jacketed thru hull risers. I have changed thermostats, but both engines still have high water pressure and gauge fluctuation. One engine still sets the high temp alarm. I am a pretty proficient mechanic, but this has got me stumped. Any suggestions ?
Click to expand...
I would verify with an IR gun the indicated temps are accurate, take the temp switch out and put in water see what temp it closes at. this woul verify if these temps are accurate or indication issue

The fact that you have higher water pressure makes me wonder if you have scale blockage etc where water is not getting out, odd as 'ell it would be on both engines though unless something odd in electrical is affecting gauge package , but both engines doubling water pressure ?

Not sure what your water flow is, but on my merc there is a T with check balls on thermostat housing that lets incoming cooling water from drive pump into the exhaust so it cools the exhaust when the thermostat is not open. Is there a similar 'bypass' that might be bleeding too much raw water out exhaust and not into the block for cooling? Doesnt mesh well with higher pressure indication though... stumped too
 
Dieter said:
I would verify with an IR gun the indicated temps are accurate, take the temp switch out and put in water see what temp it closes at. this woul verify if these temps are accurate or indication issue

The fact that you have higher water pressure makes me wonder if you have scale blockage etc where water is not getting out, odd as 'ell it would be on both engines though unless something odd in electrical is affecting gauge package , but both engines doubling water pressure ?

Not sure what your water flow is, but on my merc there is a T with check balls on thermostat housing that lets incoming cooling water from drive pump into the exhaust so it cools the exhaust when the thermostat is not open. Is there a similar 'bypass' that might be bleeding too much raw water out exhaust and not into the block for cooling? Doesnt mesh well with higher pressure indication though... stumped too
Click to expand...
It is very strange that both engines are doing the same thing. The water pressure gauges are mechanical, so that rules out electrical, at least for the WP gauges. I am going to try and flush the system out with a garden hose attached directly to the exhaust manifold water lines. If that doesn't work, I am out of ideas.
 
Dieter said:
I would verify with an IR gun the indicated temps are accurate, take the temp switch out and put in water see what temp it closes at. this woul verify if these temps are accurate or indication issue

The fact that you have higher water pressure makes me wonder if you have scale blockage etc where water is not getting out, odd as 'ell it would be on both engines though unless something odd in electrical is affecting gauge package , but both engines doubling water pressure ?

Not sure what your water flow is, but on my merc there is a T with check balls on thermostat housing that lets incoming cooling water from drive pump into the exhaust so it cools the exhaust when the thermostat is not open. Is there a similar 'bypass' that might be bleeding too much raw water out exhaust and not into the block for cooling? Doesnt mesh well with higher pressure indication though... stumped too
Click to expand...

Not sure what cooling system he has as his engines are HP blue water 502's.

The housing design you refer to (which I have on my own 1987 350 ) has the two poppet balls.

In 1987 and many other non HP years there were up to 3 different raw water cooling systems available.

Maybe he can tell us which type he has or provide a picture of the T stat housing.
 
kghost said:
Not sure what cooling system he has as his engines are HP blue water 502's.

The housing design you refer to (which I have on my own 1987 350 ) has the two poppet balls.

In 1987 and many other non HP years there were up to 3 different raw water cooling systems available.

Maybe he can tell us which type he has or provide a picture of the T stat housing.
Click to expand...
I'll get a picture this afternoon
 
Here is a picture of my thermostat housing. I know it has the spring loaded balls in the brass tee fitting
20240515_141751.jpg
 
When was the last time you took the hoses off and checked the balls?

I would start there.
The balls spin and wear and can stick closed.

Where do you live and if in the north, how do you winterize your engines?
 
Removed hoses, ran garden hose through all exhaust passages. Balls are free and look normal. Winterize using rv antifreeze pumped through running engine.
 
Ok,
After reading thru this post,
If pressure is high, that says to me that its the output thats being restricted. If pressure was low that would say input is restricted.
Do we agree on that?

Have you run this in the water yet?
Is the issue only present when running on a hose?
 
That is my assumption, but there seems to be plenty of water coming out of the exhaust pipes.
The boat is in the water. I don't ever run it very long on the hose.
 
This is like one of those Facebook posts where they ask to solve a math problem with lots of symbols for addition and multiplication but no correct structure so that the answers are all over the place. We need more from you. What exhausts are on there for one. What year were they made? You have some GLM thermostat housing there. Is that really what they came with? Who put those braided hoses on it? Those are sketchy. There seems to be more going on here than I am getting from your description.
 
Trying not to sound like a dick, but all the info you are asking for is in the original post. As far as the braided hoses, all Mercury Racing 500 HPs cam with the braided lines. As far as the GLM thermostat housing goes, it is a stock replacement. I do appreciate any constructive ideas.
 
I agree the high water pressure suggests water isn't getting out like it should. Have the exhaust elbows been checked for clogging?
Fresh water or salt water use? Manifolds and elbows, are they stainless steel, or cast iron? Exhaust flapper on the transom?
 
Manifolds are aluminum with stainless risers. I ran a garden hose through the exhaust cooling passages and there didn't seem to be any restriction. I am going to put it all back together this afternoon after flushing everything possible, with new stats and see what happens.
 
No solid answers, but it seems to be working OK now. I put in two new thermostats, along with new spacer rings. I drilled two 3/16" holes in the thermostats to make sure no air could get trapped. I also put the garden hose on every discharge hose. All seemed to be free flowing. Used an air hose and screw driver to get out any scale in the thermostat housing. Put it all back together and it seems to work fine. Not sure what it was, but it's fixed now.
 
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